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Studies Analyze New Use of End-Tidal CO2 Monitoring


We haven’t quite arrived at the Star Trek tricorder, but little by little, the miniaturization and accuracy of new, non-invasive technology is helping EMS pinpoint a variety of problems. The prehospital monitoring of end-tidal carbon dioxide (EtCO2) to date has been limited to confirmation of airway placement and monitoring of chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF) respiratory waveforms. This month, we review a new use of EtCO2 monitoring: prediction of metabolic acidosis.

Ray Fowler, MD, FACEP, a strong advocate of EMS research, is fond of calling expired CO2 the “smoke” that comes from the “fire of metabolism.” If you think of metabolism as a raging fire inside the body burning up nutrients and calories, then it would stand to reason that our bodies need to get rid of the byproduct of that fire.

One of those byproducts is the CO2 we exhale, which can be measured with non-invasive side-stream monitors. Could this technology help us determine if our metabolism isn’t functioning correctly? One group of researchers set out to prove this.

Measuring Metabolic Disturbances
Kartal M, Eray O, Rinnert S, et al. EtCO2: A predictive tool for excluding metabolic disturbances in nonintubated patients. Am J Emer Med. 2011; 29(1):65–69.

This study prospectively collected and compared EtCO2 and bicarbonate (HCO3) levels of patients seen in an urban emergency department over a three-month period. The authors considered anyone who had arterial blood gases (ABGs) drawn, a total of 399 patients. They excluded anyone with respiratory issues because these patients may be predisposed to have different lung function and EtCO2 excretion. They were able to enroll 240 patients suspected of having metabolic derangements. Could EtCO2 predict who was actually acidotic? Yes, it did.

An EtCO2 value of greater than or equal to 37 mmHg definitively ruled out acidosis (100% sensitivity, ruling out bicarbonate levels of 21 mmol/L or less). Patients with an EtCO2 value of equal to or less than 25 mmHg were highly likely to be acidotic (84% specificity). The authors also tried to see if low EtCO2 values alone could predict who would die. They found that low EtCO2 values were correlated with mortality, but that value alone didn’t predict death.

This is great news for EMS clinicians, who have access to side-stream EtCO2 readings (the so called “smart-cannulas”). Their research showed that a quick non-invasive test may give us insight into serious metabolic derangements. We can relay information to the next healthcare provider and act on that information
if appropriate.

Acute Coronary Syndrome Treatment
Selker H, Beshansky J, Ruthazer R, et al. Emergency medical service predictive instrument-aided diagnosis and treatment of acute coronary syndromes and ST-segment elevation myocardial infarction in the IMMEDIATE trial. Prehosp Emerg Care. 2011; 15(2):139–148.

In keeping with the first study, a new article reported favorable results in paramedic use of a new method to identify acute coronary syndrome (ACS) patients who need percutaneous coronary interventions (PCIs).

The researchers tested the ability of paramedics to use a new ischemia time-insensitive predictive instrument (ACI-TIPI) and thrombolytic predictive instrument (TPI) tools to improve the accuracy of diagnosis and speed of care for ACS patients.

This was part of the IMMEDIATE trial (the Immediate Myocardial Metabolic Enhancement During Initial Assessment and Treatment in Emergency Care). These authors should be applauded because they were able to work with multiple EMS agencies with great geographic and system diversity.

This was a before-and-after study model that started in 2006. I encourage you to review the full article because space constraints preclude a comprehensive review of the long-awaited paper here.

The authors reported that this new approach increased the number of EMS patients who received PCI. Using a 75% ACI-TIPI probability cutoff, they were able to justify field activation of the hospital PCI team as they approached 86% specificity. JEMS

Bottom Line

What we know: Research has repeatedly demonstrated how early recognition of
life-threatening problems by EMS leads to improved care and in-hospital outcomes.
What this study adds: Scientific evidence that an existing, non-invasive EMS tool can alert the healthcare team to possible metabolic acidosis. Look for EtCO2 values of ≤ 25 mmHg to help confirm acidosis, and values of ≥ 37 mmHg to rule it out.

This article originally appeared in May 2011 JEMS as “A Window into Metabolism: Studies analyze new use of end-tidal CO2 monitoring.”


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